This invention relates generally to electron emitters. In particular, the invention relates generally to cold electron emitters of p-n cathode type.
Electron emission technology exists in many forms today. Hot cathode ray tubes (CRT), where electrons are produces as a result of thermal emission from hot cathode heated by electrical current, are prevalent in many displays such as televisions (TV) and computer monitors. Electron emission also plays a critical role in devices such as x-ray machines and electron microscopes. Miniature cold cathodes may be used for integrated circuits and flat display units. In addition, high-current density emitted electrons may be used to sputter or melt some materials.
In general, two types of electron emitters existxe2x80x94xe2x80x9chotxe2x80x9d and xe2x80x9ccoldxe2x80x9d cathode emitters. The xe2x80x9chotxe2x80x9d cathodes are based on thermal electron emission from surface heated by electric current. The cold cathodes can be subdivided into two different types: type A and B. The emitters of type A are based on the field emission effect (field-emission cathodes). The emitters of type B are the p-n cathodes using the emission of non-equilibrium electrons generated by injection or avalanche electrical breakdown processes.
Both types of emitters have drawbacks which make them virtually impractical. For type A emitters (field emission type), one of the main drawbacks is their very short lifetime. For example, the type A emitters may be operational for just hours, and perhaps even as short as minutes. In the cold field-emission cathodes (type A), electrons are extracted from the surface of a metal electrode by a strong electric field in vacuum. The field cathodes have a short lifetime at large emitted currents, which are needed in recording devices and other applications.
With reference to FIG. 1A, operation of type A emitters will be described. FIG. 1A illustrates a typical energy diagram for a metallic surface illustrating a concept of a work function of a metal. As shown, a material, in this instance a metal, is on the left and a vacuum region is on the right. EF represents a Fermi level of the metal. The work function of the metal "PHgr"M is the energy required to move a single electron from the Fermi level in the metal into vacuum. Thus, the work function "PHgr"M is the difference between Vac and EF. The work function "PHgr"M for metal is typically between 4-5 electron volts (eV).
In very strong external field the energy diagram changes, and it looks as a triangular potential barrier for the electrons (FIG. 1A, dashed line). When the external field F increases, the barrier width decreases and the tunneling probability for electrons rapidly increases. The transparency of such a barrier is       D    =          exp      ⁡              [                  -                                    4              ⁢                              Φ                M                                  3                  /                  2                                            ⁢                                                2                  ⁢                  m                                                                    3              ⁢              qhF                                      ]              ,
where F the electric field, q and m are the electron charge and mass. Transparency represents the probability of electron tunneling. For current densities j=1-100 A/cm2 (amperes per square centimeter) the corresponding field would be F greater than 107 V/cm.
In such strong fields, the ions, which are always present in a vacuum region in actual devices, acquire the energy over 103 eV in the vacuum region on the order of one micron or larger. Ions with such strong energies collide with the emitter surface leading to absorption of the ions and erosion of the emitter surface. The ion absorption and erosion typically limits the lifetime of type A emitters to a few hours of operation or even to a few minutes. Damage to cathodes in systems with the fields of similar strength has been studied in great detail and is rather dramatic.
For type B emitters (injection/avalanche type), one of the main drawbacks is that the efficiency is very small. In other words, the ratio of emitted current to the total current in the circuit is very low, usually much less than 1%. The cathode of type B based either on p-n junctions, or semiconductor-metal (S-M) junction including TiO2 or porous Si, or the avalanche electrical breakdown need an xe2x80x9cinternalxe2x80x9d bias, applied to p-n junction or S-M junction.
Alternatively, there have been suggestions to use the electrical breakdown processes to manufacture the cold emitters from Si. These types of avalanche emitters are based on emission of very hot electrons (with energies of the order of a few electron volts) accelerated by very strong electric field in the avalanche regime. As a result, they also have a disadvantage that the emitted current density of the hot electrons is very small.
Attempts have been made to increase the current density by depositing cesium (Cs) on semiconductor surface to use a negative electron affinity (NEA) effect. FIG. 1B illustrates the concept of NEA. As shown, a material, a p-type semiconductor in this instance, is on the left and a vacuum region is on the right. EC represents a conduction band of the metal. Note that the NEA effect corresponds to a situation when the bottom of the conduction band EC lies above the vacuum level Vac. One earlier p-n cathode of this type combined a silicon, or gallium arsenide avalanche region, with cesium metallic layer from where the emission took place (GaAs/Cs or GaP/Cs structures). However, Cs is a very reactive and volatile element. Thus, the GaAs and GaP emitters with Cs are not stable at high current densities.
In short, cold emitters with both high current emission and stability were not possible with previous designs.
In one respect, an embodiment of a cold electron emitter may include an heavily doped n-type region (n+ region). The n+ region may be formed from wide band gap semiconductors. The electron emitter may also include a substrate below the n+ region. Indeed, the n+ region may be formed by doping the substrate with electron rich materials. In addition, the electron emitter may include a p region formed within or above the n+ region. The p region may be formed by counter doping the n+ region with electron poor materials. The thickness of the p region is preferred to be less than the diffusion length of the electrons in the p region. Also, the hole concentration level in the p region is preferred to be less than the electron concentration in the n+ region. The electron emitter may further include a metallic layer formed above the p region. The work function of the metallic layer is preferred to be less than the energy gap of the p region. In addition, the thickness of the metallic layer is preferred to be on the order of or less than the mean free path for electron energy. The electron emitter may still further include a heavily doped p region (p+ region) formed within the p region, for example, by delta-doping the p region. The electron emitter may yet further include n and p electrodes so that n+-p junction may be forward biased for operation, for example, to control the amount of current emitted from the device. The electron emitter may still yet further include an M electrode, with or without the p electrode.
In another respect, an embodiment of a method to fabricate an electron emitter may include forming an n+ region, for example, from doping a wide band gap substrate with electron rich materials. The method may also include forming a p region within the n+ region, for example, by counter doping the n+ region with electron poor materials. The thickness of the p region is preferred to be less than the diffusion length of the electrons in the p region. Also, the hole concentration level in the p region is preferred to be less than the electron concentration of the n+ region. The method may further include forming a metallic layer above the p region. The work function of the metallic layer is preferred to be less than the energy gap of the p region, and the thickness of the metallic layer is preferred to be of the order of or less than the mean free path for electron energy. The method may still further include forming a p+ region, for example, by delta doping the p region. The method may yet include forming n and p electrodes so that n+-p junction may be forward biased for operation. The method may yet further include forming an M electrode, with or without forming the p electrode, to control-the amount of current emitted from the current emitter.
The above disclosed embodiments may be capable of achieving certain aspects. For example, the electron emitter may produce high density of emitted electron current. Also, the lifetime of the emitter may be relatively high. Further, the emitter may be based on well-known wide-gap materials and fabrication methods there of and thus, little to no capital investment is required beyond that present in the current state-of-the-art. In addition, the detrimental effects of high vacuum fieldxe2x80x94cathode surface erosion, ion absorption at the emitter surface, etc.xe2x80x94may be avoided since the device does not require strong electric fields in vacuum region, which results in stable operation. Thus, stability and high current density may be combined in a single device. The absence of need to use high fields in vacuum region may significantly simplify packaging, which would not require a high vacuum.
In short, unlike the prior devices, at least some embodiments of the present invention allows for cold durable emitters with large emitted currents and large efficiency.